Engines of the type having a rocker arm in a valve mechanism draw in and discharge a fuel gas or an exhaust gas by pressing a valve end to open the valve with an adjustment screw on the distal end of a rocker arm that is actuated by a cam. When the rocker arm returns to its original position, the valve is closed again under the resiliency of a spring.
A clearance (hereinafter referred to as a tappet clearance) is provided between the valve end and the adjustment screw, for allowing the valve to be fully closed when the rocker arm returns to its original position. If the tappet clearance is too small, then the clearance may possibly be eliminated due to thermal expansion occurring at high temperatures. If the tappet clearance is too large, then the valve end and the adjustment screw produce large sounds as noise upon contact with each other. Therefore, the tappet clearance has to be adjusted accurately to an appropriate value (or an appropriate range), which is preset in design. Particularly, a process for manufacturing a large quantity of engines in a wide variety of types needs to have a reduced adjustment time per engine, while maintaining a high adjustment accuracy level. It is preferable to be able to adjust the tappet clearance automatically in order to prevent adjustment fluctuations.
In view of the foregoing, there has been proposed an apparatus for automatically adjusting a tappet clearance using an adjustment screw rotating mechanism, a cam shaft rotating mechanism, a displacement sensor for detecting an amount of valve lift, and a processing device (see, Japanese Laid-Open Patent Publication No. 7-109909).
The applicant of the present application has proposed a tappet clearance adjusting apparatus for detecting when a valve is brought into contact with a valve seat by detecting a torque value for rotating an adjustment screw, and for adjusting the tappet clearance quickly and highly accurately (see Japanese Patent Application No. 2004-283089). The adjusting apparatus should preferably have an adjustment screw and a screwdriver, which are held in proper coaxial engagement with each other, due to the need for highly accurately detecting a change in the torque value, at a time when the valve head is brought into contact with the valve seat. Therefore, an adjustment unit including a motor and the adjustment screw has an angle thereof set highly accurately by a robot.
However, since the angle of the adjustment screw varies depending on the amount of lift of the valve, a procedure for operating the robot becomes complex, in order to cause the robot to respond to changes in the angle of the adjustment screw. The adjustment screw may not necessarily be set at an appropriate direction, due to slight operational delays of the robot or the like.
For directing the screwdriver appropriately with respect to the adjustment screw, it may be proposed to use a bendable torque transmitting mechanism, such as the fastening device disclosed in Japanese Laid-Open Patent Publication No. 6-39655, for example.
However, the fastening device disclosed in Japanese Laid-Open Patent Publication No. 6-39655 includes a rotary mechanism at a bent tip portion thereof, which is too complex to rotate smoothly for detecting rotational torque with high accuracy. Even if the fastening device is able to hold the screwdriver in engagement with the adjustment screw and at an appropriate direction, the fastening device cannot be used to adjust the tappet clearance, since it is devoid of any mechanism for rotating the adjustment nut.